The ApoE4 genotype is the strongest genetic risk factor for developing AD. However, the mechanisms that underlie this link between ApoE4 genotype and AD are not well understood. Objective/Hypothesis: the objectives of this proposal are to understand the molecular underpinnings of the association between ApoE4 genotype-specific changes in brain phospholipid homeostasis and ApoE4 increased susceptibility to develop late-onset AD. Our preliminary data indicate that the levels of PI(4,5)P2 are reduced in postmortem human brain tissues of ApoE4 carriers, in the brain of ApoE4 homozygous knock-in (KI) mice, and in primary neurons expressing ApoE4 alleles, if compared to ApoE3 counterparts. The expression of synaptojanin 1 (synj1) that dephosphorylates PI(4,5)P2 reducing its levels, is elevated in ApoE4 brains. Our recent observations demonstrate that synj1 reduction (with subsequent elevation of PI(4,5)P2 levels) can accelerate endosomal/lysosomal degradation of A?nd ameliorate cognitive deficits in AD transgenic mice. In this proposal we are testing the hypothesis that ApoE genotype is a critical determinant of brain phospholipid homeostasis and that the ApoE4 isoform is dysfunctional in this process (increased synj1 expression and reduced PIP2 levels). As a consequence, ApoE4 impairs A?learance through endosomal/lysosomal degradation pathway, accelerates cognitive decline, and disrupts synaptic functions. These ApoE4-induced changes in the cascade of aberrant molecular events lead to long-term neurodegenerative process and AD development. Rationale/Experimental Design: In this application, we will study whether reducing synj1 thus normalizing brain phospholipid metabolism can rescue ApoE4-related neuropathological changes by utilizing mouse models of synj1 haploinsufficiency with human ApoE4 or E3 homozygous KI background in studies that assess: 1) AD-related cognitive dysfunction (aim 1.1); 2) AD-related biochemical changes such as A?learance and ApoE secretion (aim 1.2 and 1.3); 3) AD related morphological changes and synaptic phospholipid homeostasis (aim 2); 4) molecular mechanisms underlying ApoE isoform specific changes in synj1 expression/PIP2 homeostasis (aim 3). Relevance/Impact: The proposed studies in this application will be the first mechanistic studies that link ApoE4 genotype-specific changes in brain phospholipid homeostasis to ApoE4 increased susceptibility to develop AD. These studies may uncover new therapeutic options for the treatment of AD targeting at ApoE4 pathogenic nature.